elasticsearch/elasticsearch — pull request #3963 by s1monw (2013-10-24 12:04:27+00:00)
Title: Use abstract classes as super typs for Acknowledge operations

Currently we have a marker interface for Acknowledged[Request|Response],
this makes not much sense since we duplicate the code in each subclass
or class that implements the interface. We can simply use abstract
classes and have it implemented only once.